1) Field of the Invention
This invention relates to colored oxygen scavenging polymers and articles made from such polymers. In particular it relates to polymer blends containing a base polymer, an oxidizable organic polymer, a transition metal catalyst, and a colorant. The colorant is selected from a group of colorants that do not completely deactivate the transition metal catalyst. A preferred colorant, yields in an article made from the melt polymer blend, a Catalyst Deactivation Factor (CDF) of less than about 0.25, preferably less than 0.15, more preferably less than 0.1, and most preferred less than 0.05. The present invention also comprises a colored monolayer article made from such polymer blends having the described CDF, such as a film, thermoformed tray, or blow molded container, that has active oxygen scavenging properties.
2) Prior Art
Typical polymers used in making film, thermoformed trays, or blow molded containers, are primarily based on polyester due to its physical properties. Suitable polyesters can be homopolymers such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), or copolymers of either or both. For blow molded containers, polyethylene terephthalate isophthalate copolyester (PET/IP) is particularly useful.
Oxygen scavenging polymers are well known and are especially useful in the food packaging business. It is known that oxygen can have an effect on the odor and taste of packaged food thereby shortening the shelf life of the food. Oxygen scavenging packaging materials, on the other hand, react with oxygen that is in the process of traversing the packaging barrier. Thus the oxygen scavenging packaging material reduces or eliminates the odor and/or undesirable taste of food or beverages excessively exposed to oxygen.
Typical oxygen scavenging compounds are oxidizable organic polymer molecules containing allylic positions such as polybutadiene based polymers, or polyethylene/cyclohexene copolymers, or containing benzylic positions such as m-xylylamine-based polyamides, or mixtures of these. The use of oxidizable organic polymers by themselves results in a very slow oxidative process, but such polymers lack the desired physical properties of PET, for example, and are very costly compared with PET. The incorporation of oxidation catalyst into the oxidizable polymer solves this problem.
With respect to the oxidizable organic polymers mentioned earlier, poly(m-xylylene adipamide) (known commercially as MXD6) is widely known. Additionally, the prior art discloses that the oxidizable organic polymers need a transition metal catalysts to make it actively scavenge oxygen. The most common transition catalyst described by the prior art is a cobalt salt.
PCT Patent Application WO 98/12244 in the name of inventors Cahill et al. and assigned to Amoco Corporation discloses an oxygen scavenging composition wherein the critical oxidizable organic polymer is polybutadiene and the catalyst for the oxidizable organic polymer is transition metal salts or other compounds. This application discloses the usefulness of such a composition as a blend with polymers for packaging films and containers for the food and beverage industry.
PCT Patent Application WO 99/48963 in the name of Ching et al. and assigned to Chevron Chemical Company discloses an oxygen scavenging composition including a polymer or oligomer having at least one cyclohexene group or functionality. This application also discloses the use of transition elements as a catalyst to activate the oxygen scavenging composition. The transition metal catalysts are employed in the form of salts and other compositions. This reference also notes that cobalt, a transition metal catalyst, is preferred.
The use of a transition metal catalyst to promote oxygen scavenging in polyamide multilayer containers, and blends with polyethylene terephthalate (PET), has been disclosed in the following patents, for example.
U.S. Pat. Nos. 5,021,515, 5,639,815 and 5,955,527 to Cochran et al. disclose the use of a cobalt salt as the preferred transition metal catalyst and poly(m-xylylene adipamide) (MXD6) as the preferred oxidizable organic polymer.
U.S. Pat. Nos. 5,281,360 and 5,866,649 to Hong, and U.S. Pat. No. 6,288,161 to Kim also discloses blends of MXD6 with PET and a cobalt salt catalyst.
Until recently these oxidizable organic polymers have been used as an inner layer in multilayer films, thermoformed trays or blow molded containers. For colored articles, such as green, blue or amber bottles, the colorant is mixed in the non-scavenging outer and inner layers. In such multilayer articles there is no reaction between the colorant and the transition metal catalyst contained in the oxygen scavenging layer. There is a need for colored monolayer films, sheets and containers to reduce the cost of multilayer processes.
It is generally accepted that the oxidation of polymers involves a free radical reaction with the formation of hydroperoxides. Transition metal ions catalyze the decomposition of hydroperoxides to radical species that greatly accelerate the rate of oxidation and thus the rate of oxygen scavenging.
Surprising it has been found that certain colorants deactivate the catalyst, after melt blending and thereafter (such as in an article), making them less effective as a catalyst. Thus only certain colorants can be used for monolayer oxygen scavenging articles since the colorant is intimately mixed in a melt phase with the transition metal catalyst, unlike prior multilayer film, thermoformed trays, or blow molded containers.